In the Dark

Yesterday evening (10pm Irish Time) saw the release of new results from the Dark Energy Spectroscopic Instrument (DESI), completing a trio of major announcements of cosmological results in the space of two days (the Atacama Cosmology Telescope and the Euclid Q1 release being the others). I didn’t see the DESI press conference but you can read the press release here.

There were no fewer than eight DESI papers on the astro-ph section of the arXiv this morning. Here are the titles with links:

• DESI DR2 Results I: Baryon Acoustic Oscillations from the Lyman Alpha Forest
• DESI DR2 Results II: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints;
• Construction of the Damped Lyα Absorber Catalog for DESI DR2 Lyα BAO;
• Validation of the DESI DR2 Lyα BAO analysis using synthetic datasets;
• Validation of the DESI DR2 Measurements of Baryon Acoustic Oscillations from Galaxies and Quasars;
• Extended Dark Energy analysis using DESI DR2 BAO measurements;
• Constraints on Neutrino Physics from DESI DR2 BAO and DR1 Full Shape;
• Data Release 1 of the Dark Energy Spectroscopic Instrument.

You can see from the titles that the first seven of these relate to the second data release (DR2; three years of data) from DESI; the last one listed here is a description of the first data release (DR1), which is now publicly available.

Obviously there is a lot of information to digest in these papers so here are two members of the DESI collaboration talking with Shaun Hotchkiss on Cosmology Talks about the key messages from the analysis of Baryon Acoustic Oscillations (the BAO in the titles of the new papers):

A lot has been made in the press coverage of these results about the evidence that the standard cosmological model is incomplete; see, e.g., here. Here are a few comments.

As I see it, taken on their own, the DESI BAO results are broadly consistent with the ΛCDM model as specified by the parameters determined by the Cosmic Microwave Background (CMB) inferred from Planck. Issues do emerge, however, when these results are combined with other data sets. The most intriguing of these arises with the dark energy contribution. The simplest interpretation of dark energy is that it is a cosmological constant (usually called Λ) which – as explained here – corresponds to a perfect fluid with an equation-of-state p=wρc² with w=-1. In this case the effective mass density of the dark energy ρ remains constant as the universe expands. To parametrise departures from this constant behaviour, cosmologists have replaced this form with the form w(a)=w₀+w_a(1-a) where a(t) is the cosmic scale factor. A cosmological constant Λ would correspond to a point (w₀=-1, w_a=0) in the plane defined by these parameters, but the only requirement for dark energy to result in cosmic acceleration is that w<-1/3, not that w=-1.

The DESI team allow (w₀, w_a) to act as free parameters and let the DESI data constrain them, either alone or in combinations with other data sets, finding evidence for departures from the “standard values”. Here’s an example plot:

The DESI data don’t include the standard point (at the intersection of the two dashed lines) but the discrepancy gets worse when other data (such as supernovae and CMB) are folded in, as in this picture. The weight of evidence suggests a dark energy contribution which is decreasing with time.

These results are certainly intriguing, and a lot of credit is due to the DESI collaboration for working so hard to identify and remove possible systematics in the analysis (see the papers above) but what do they tell us about ΛCDM?

My view is that we’ve never known what the dark energy actually is or why it is so large that it represents 70% of the overall energy density of the Universe. The Λ in ΛCDM is really just a place-holder, not there for any compelling physical reason but because it is the simplest way of accounting for the observations. In other words, it’s what it is because of Occam’s Razor and nothing more. As with any working hypothesis, the standard cosmological model will get updated whenever new information comes to light (as it is doing now) and/or if we get new physical insights into the origin of dark energy.

Do the latest observations cast doubt on the standard model? I’d say no. We’re seeing an evolutionary change from “We have no idea what the dark energy is but we think it might be a cosmological constant” to “We still have no idea what the dark energy is but we think it might not be a cosmological constant”.